[quote name='Maklershed']So far CAG members have not steered me in the wrong direction. . .[/QUOTE]
Apparently, you've been quite fortunate so far because with the ever-growing 20,000+ membership have come quite a few crackpots.
With that said, you're mainly looking at the Nintendo brand exclusives so far. If you're looking for family-friendly titles, anything with "Mario" in the title is probably worth owning for GCN.
If you're looking for something more mature, you might try REmake, MGS: Twin Snakes, Eternal Darkness, etc. Several others have already mentioned the decent RPGs (Tales of Symphonia, Skies of Arcadia: Legends, Baten Kaitos, etc.).
Unfortunately, most of the once-exclusive third party titles have gone multi-console (Viewtiful Joe, RE4, Monkey Ball series), so you might just want to check around your local game shops for any remaining exclusives that interest you.